1. Field of the Invention
The present invention relates to devices for centering and rotationally aligning a flatted object to a desired orientation.
2. Description of Related Art
Microelectronic devices are typically constructed on a wafer having a substantially circular perimeter. During each processing step, the wafer must be correctly aligned by centering the wafer and turning it to the desired rotational orientation. This allows each processing step to proceed in alignment with the work done on the wafer during previous processing steps. The centering is done with respect to the circular perimeter of the wafer, however, in order to detect the correct rotational orientation, the circular perimeter of the wafer must be marked.
One method of marking the circular perimeter of the wafer is with a flatted chord portion. The flatted chord defines a unique rotational orientation for each wafer that is used to consistently align the wafer each time it is moved. Typically, the wafer will be moved into a desired position by a transport mechanism. The wafer is usually aligned into a known starting orientation before the transport mechanism moves the wafer. This alignment is conventionally achieved with an active alignment device that centers and rotates each wafer to the starting orientation so that the transport mechanism can then move it into the final aligned position.
Once the wafer has been placed in a processing chamber with the desired orientation, the wafer is clamped, typically with an electrostatic chuck. To ensure that each wafer is correctly processed, it is critical that the electrostatic chuck remain clean and uncontaminated. During the processing of a wafer, the electrostatic chuck is covered and protected against contamination by the overlying wafer. However, when the interior of the processing chamber is being cleaned, there is no wafer present. To protect the electrostatic chuck during such cleaning, a protective electrostatic chuck (PEC) cover is placed on the chuck.
The PEC cover has the same shape as the wafer, and it too must be correctly aligned relative to the electrostatic chuck to ensure that the PEC cover completely protects the chuck during the cleaning operation. Like the wafers, the PEC cover is conventionally aligned with an active alignment device prior to being moved by the transport system into the processing chamber and onto the electrostatic chuck.
An active alignment device uses a motorized drive system to position the wafer with the desired predetermined angle and position. This predetermined angle and position ensures that the wafer is picked up with a known orientation by the transport mechanism, which will then be able to deposit it with the desired final orientation for processing. However, active alignment devices are expensive. Accordingly, there is a need for a low cost alignment device, preferably one that operates passively and is driven by the weight of the wafer or PEC cover under the influence of gravity.
Active alignment devices that use motors are also relatively large. Although the wafers can be pre-aligned before they are moved into a processing system, the PEC cover is often stored internally where there is limited space available. Although the transport mechanism can accurately position a pre-aligned wafer or PEC cover on the chuck, each time the object is picked up and set down there is some positioning error. This positioning error can build up to unacceptable levels for a PEC cover that is repeatedly handled if it is not aligned between uses.
Consequently, a large active alignment mechanism that is suitable for pre-aligning wafers before entering the processing system may not be suitable for a PEC cover stored internally that must be repeatedly handled and repositioned accurately. A small, inexpensive aligner for PEC covers is particularly desirable.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an alignment device for aligning a flatted object to a desired orientation that is low cost, has a small size and preferably operates passively to bring the flatted object to a desired orientation under the influence of gravity.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention, which is directed to an alignment device for aligning a flatted object to a desired orientation. The flatted object has a substantially circular perimeter and a flatted chord portion and the alignment device includes a base, first and second conically shaped centering rollers and at least one conically shaped rotational alignment roller.
Each conically shaped roller is rotationally mounted to the base on a vertical roller axis and includes a bottom and a top. The roller is oriented such that the bottom of the conical portion has a greater diameter than the top. The bottom of each roller has a circular perimeter and the bottom perimeters of all the rollers define the final location and desired orientation of the flatted object.
The first and second centering rollers are mounted to the base such that the circular perimeters of the bottoms of the first and second conical rollers are substantially tangential to the desired final location for the circular perimeter of the flatted object when the flatted object is in the desired position. Each rotational alignment roller is mounted to the base such that the circular perimeter of its bottom is substantially tangential to the flatted chord portion of the flatted object when the flatted object is in the desired rotational orientation.
In the most highly preferred embodiment of the invention, the alignment device includes two rotational alignment rollers. Each rotational alignment roller is mounted such that the circular perimeter of its bottom is substantially tangential to the flatted chord portion of the flatted object when the flatted object is in the desired orientation. The preferred location for the two rotational alignment rollers is at opposite ends of the flatted chord portion of the flatted object.
It is preferred that all the rollers turn freely so that the flatted object will automatically and passively align itself to the desired position and rotational orientation under the influence of gravity. However, one or more of the rollers may be motorized, if desired, with sensors employed to detect when the desired orientation has been achieved.
Each conically shaped roller also includes a bottom lip for supporting the flatted object. The bottom lip has a diameter greater than the diameter of the bottom of its corresponding roller.
The conical portion of each roller has a surface sufficiently smooth and a conical angle sufficiently steep that the flatted object will slide down the surface into substantially tangential contact with the bottom of the conically shaped roller. A conical angle of at least fifty and preferably about seventy degrees is preferred. A suitable and preferred material for the rollers is a ceramic, such as an aluminum oxide ceramic.
The first and second rotational alignment rollers define a midpoint therebetween. It is preferred that this midpoint and the first and second centering rollers be approximately equally spaced from each other around the perimeter of the flatted object.